1. Field of the Invention
The present invention relates to a method for manufacturing an artificial leather, and more particularly to a method for manufacturing an artificial leather reinforced with ultramicrofiber nonwoven fabric.
2. Description of the Related Art
Typically, artificial leathers containing microfine fibers are manufactured by conjugate spinning. The microfiber made by conjugate spinning has a fineness of at most 0.015 denier; therefore, artificial leathers containing the microfibers lack suede-like touch. Artificial leathers containing microfibers manufactured by blend spinning have suede-like appearance and silky touch, but the strength of leather is quite weak. Artificial leathers manufactured by impregnating ultramicrofibers with polyurethane (PU) resin composition are well known. A method disclosed in Taiwan Patent Publication No. 152961 (Application No. 78107985), entitled “Genuine-leather-like Complex Ultramicrofiber nonwoven fabric”, is shown below.

However, there is a big difference between the longitudinal elongation and the transverse elongation of the conventional microfiber nonwoven. Since the transverse elongation is much higher than the longitudinal elongation, the use of the microfiber nonwoven is limited by the orientation issue. Therefore, this nonwoven is not suitable for use in manufacturing artificial leathers.
Furthermore, microfiber nonwovens manufactured by spunlace typically have distinct traces of the waterjet and a lower peeling strength. Therefore, they are not suitable for use in manufacturing artificial leathers.
The methods of producing fibers by conjugate spinning and producing microfibers from those fibers are well known. In the conjugate spinning process, at least two compatible polymers are extruded into a fiber, and one polymer component of the resultant fiber is selectively dissolved thereby obtaining microfibers.
Besides, an artificial leather which is reinforced by a woven fabric is well known. For example, Japanese Examined Patent Publication (Kokoku) No. 4-1113 discloses a method of using a woven fabric made of high twist (above 1000 T/m) yarn as a reinforcement substrate. This technique is used to manufacture a soft artificial leather with high strength by needlepunching a staple fiber nonwoven and the reinforcement substrate into an integrated structure while minimizing damage to the reinforcement substrate by the needlepunch. The integrated structure of the staple fiber nonwoven and the reinforcement substrate are shrunk a little and impregnated with polyvinyl alcohol. Then, one part of the major fibers has a portion extracted and removed, and the product thereof is impregnated with an elastomer resin to produce artificial leather.
Also, Japanese Unexamined Patent Publication (Kokai) No. 55-57059 discloses a method of utilizing high-pressure water flow to entangle a laminate of reinforcement fabric made of shrinkable filaments with a paper nonwoven made of staple fibers (fiber length<10 mm) so as to form the artificial leather, and then shrinking the reinforcement fabric so as to increase the surface fiber density of the artificial leather.
Each of the aforementioned techniques has advantages and disadvantages. It is the purpose of the technique disclosed in the Japanese Examined Patent Publication (Kokoku) No. 4-1113 to produce a flexible substrate with a higher cutting strength by integrating the reinforcement fabric with the microfiber nonwoven. The artificial leather made from the aforementioned substrate has a higher strength while being cut, and has a significantly lower deformability due to the reinforcement fabric, as compared to other artificial leathers made from only nonwoven. However, the artificial leather still has a relatively high deformability in the range of about 10%. In the aforementioned method, though the damages occurred during needlepunch can be minimized and the artificial leather can be strengthened after the entangling step, the surface density of the entangled nonwoven is not acceptable. In order to increase the surface density, the entangled nonwoven is typically subjected to a heat shrinking treatment. However, in the technique disclosed in the aformentioned publication, since the nonwoven lacks for shrinkability, the heat treatment can only result in slight shape change which is equal to the torque release of the high twist filaments of the reinforcement fabric. Actually, if one of the nonwoven or the fabric is subjected to the heat treatment and then shrinks in order to increase the surface density of the laminate of the nonwoven and the fabric, the heat treatment will also cause the laminated artificial leather to curl, wrinkle or have an uneven surface which can cause problems in subsequent steps and degrade the quality of product. When a three-layer nonwoven/fabric/nonwoven laminated structure is adopted, the structure can be shrunk to a degree. However, the shrinking degree is too small to increase the surface density. Furthermore, PVA (polyvinyl alcohol) impregnation is disclosed in the aforementioned publication. However, when PVA impregnation is conducted, the space between fibers is filled with PVA such that the space between the fibers is enlarged thereby lowering the surface density of the artificial leather made from the fibers impregnated with PVA. Also, the artificial leather is deformed easily while being stretched.
The technique disclosed in Japanese Unexamined Patent Publication No. 55-57059 is directed to using waterjet to entangle a reinforcement fabric made of shrinkable filaments with a paper nonwoven made of staple fibers (fiber length<10 mm). However, the entanglement between the fibers of the nonwoven is quite small and the entanglement between the nonwoven and the reinforced fabric is also small. Besides, only a small amount of elastomer resin is adhered thereby contributing little anti-deformability to the nonwoven when the artificial leather is deformed. Furthermore, the products of this method have a disadvantage that the fibers are prone to drop from the artificial leather.
In another method, after a nonwoven is entangled, a reinforcement fabric is attached onto the backside surface of the entangled nonwoven. When a shrinkable nonwoven is used, the artificial leather with a higher surface density can be obtained. However, since the bonding strength between the nonwoven and the reinforcement fabric is quite weak, delamination is observed during subsequent processing steps. Furthermore, this method needs an attaching step thereby increasing the manufacturing cost.
In order to manufacture artificial leathers with a high anti-deformability (i.e., a smaller difference between the longitudinal elongation and the transverse elongation thereof), the bonding of the nonwoven to the reinforcement fabric is preferably conducted by laminating the nonwoven having a low degree of entanglement on the reinforcement fabric. Preferably, the nonwoven and the reinforcement fabric are firmly entangled in a mechanical way by spunlace or needlepunch. Thus, by laminating with the entangled reinforcement fabric (especially adequate selection of the reinforcement fabric), the finished artificial leather can be endowed with anti-deformability and the shape change (especially the elongation due to the tensile force in the manufacturing process) of the artificial leather occurred in most of the manufacturing steps can be minimized. In the process of manufacturing a suede-like artificial leather, the surface fiber density is the most important factor to determine the grade of the product.
The surface density of the complex artificial leather can be increased by two important ways including (1) shrinking the nonwoven to increase the surface density; and (2) minimizing the elongation due to the tensile force in the manufacturing process. However, as mentioned above, high-surface-density artificial leather with minimized shape change (especially the elongation due to the tensile force in the manufacturing process) cannot be obtained by current methods.